1. Field of Invention
The present invention relates to mapping the open natural fractures in the hydrocarbon reservoirs, using two surface-generated seismic signals by measuring their elastically nonlinear interaction caused due to their transmission through the open fractures.
2. Description of the Prior Art
In most of the carbonate and certain sandstone reservoirs, natural fractures are encountered that are open and control the directional permeability and the effective flow pathways for the hydrocarbons. Mapping these fractures and their orientation is the key to the economic recovery of hydrocarbons from these reservoirs. At present, natural fracture characterization is of increasing importance, since the industry is venturing into increasing their producible reserves from the existing fields that are showing production decline.
Natural fractures in the subsurface rocks are usually vertical and are mostly found in the formations that have gone through structural deformation or have experienced regional stresses. These fractures commonly terminate at lithologic discontinuities within the reservoir formations. These fractures can be closely or widely spaced and irregularly distributed. Quite often, swarms of fractures are encountered with unfractured intervals in between. Economic hydrocarbon production from the fractured reservoirs requires an optimal access of the wellbore to the open fractures. This makes it extremely important that an accurate map of the open fracture system should be available prior to any field infill and development program.
In many cases, fractures are difficult or impossible to map adequately by using currently available technologies. Physical measurements through cores and well logs are limited to the vicinity of the wells drilled in the reservoir. The density of sampling the reservoir rock using cores and well logs quite often is not sufficient to provide any useful information regarding the orientation and the location of the fractures. This is due to two main characteristics of the majority of the wells that are drilled:
1) Both the wells and the fractures are generally vertical and parallel to each other; and
2) The wellbore is smaller than the fracture spacing between the larger fractures.
Horizontal drillingxe2x80x94where the cost of drilling a well is highxe2x80x94has to be designed to take full advantage of the natural fractures that are open, by mapping their location and their orientation. Since a single horizontal well is limited in producing from a few layers of the reservoir, it is important to identify the part of the reservoir from which the production can be optimized, prior to drilling the well. This requires that the specific fractured beds should be identified prior to any drilling commitments.
This invention uses the elastically nonlinear interaction between two seismic waves as they propagate through the open fractures in the reservoir formation. Two compressional seismic signals are used. One is a higher frequency swept signal (xe2x80x98carrierxe2x80x99 wave) transmitted from the surface, using a surface seismic vibratory source. This xe2x80x98carrierxe2x80x99 wave penetrates the reservoir and travels through the fractured rock and after being reflected from the formation directly below the fractures, is recorded by the receivers that are located on the surface. The second is a lower-frequency seismic signal (xe2x80x98modulationxe2x80x99 wave) that is also transmitted from the surface using a similar source like a surface seismic vibrator. The nonlinear elastic interaction between the xe2x80x98carrierxe2x80x99 wave and the xe2x80x98modulationxe2x80x99 wave, as both the waves travel through the open fracture, is measured.
The nonlinear interaction between the two compressional waves, mentioned above, will be zero when the xe2x80x98modulationxe2x80x99 source and the xe2x80x98carrierxe2x80x99 source are located parallel to the fractures or directly above them. The interaction will also be zero when the sources and the receivers are located on the surface in such a manner that the recorded reflected signal does not intersect any subsurface open fractures.
By moving the surface sources to different locations on the surface and recording the reflected seismic signals from the subsurface formations by multiple receivers located on the surface, the measurements of the nonlinear interaction between the xe2x80x98carrierxe2x80x99 and xe2x80x98modulationxe2x80x99 waves can be used to determine the location and orientation of the fractures.
Since practically all the subsurface fractures are vertical, the fracture width of the open fractures is not modulated when the xe2x80x98modulationxe2x80x99 and the xe2x80x98carrierxe2x80x99 surface sources are directly above them. The xe2x80x98modulationxe2x80x99 is maximized when the xe2x80x98modulationxe2x80x99 source is at or near right angles to the fractures and at a distant offset, so that the xe2x80x98modulationxe2x80x99 seismic signal is arriving at the fracture at a wide angle.
In a three dimensional (3-D) seismic recording, the surface receivers are located over a large surface area, providing a good distribution of source/receiver offset distances. Multiple receiver layout configurations are used to provide an even distribution of source/receiver azimuthal angles for the reflection paths of the seismic signals. The art and knowledge to use different configurations of seismic sources and receivers for 3-D recording are well known in the seismic-imaging industry, and need not be described in detail in this invention. Once the main concept of this invention is understood, anyone familiar with 3-D seismic recording can use this invention to map the orientation and location of the open fractures by analyzing the data representing the nonlinear interaction between the two transmitted waves, recorded by receivers over large distribution of source/receiver azimuthal angles and offset distances.
Briefly, the present invention provides a new and accurate seismic method of mapping the orientation and location of the open natural fractures that are common in the hydrocarbon reservoirs. Two predetermined seismic signals are used. One is a higher frequency swept signal referred to in the description as a xe2x80x98carrierxe2x80x99 signal that is in a higher frequency range compared to the lower frequency signal, which is termed as a xe2x80x98modulationxe2x80x99 signal. The xe2x80x98carrierxe2x80x99 signal is transmitted using a vibratory surface seismic source located in a pattern designed for conventional 3-D seismic recording. The lower frequency source is also located on the surface and can be easily deployed in any geometric pattern that is considered necessary to map the location and orientation of the fractures. For this particular description both the seismic surface sources that generate the xe2x80x98modulationxe2x80x99 and the xe2x80x98carrierxe2x80x99 seismic signals are located at the same surface location. Since the lower frequencies are less attenuated as they travel through the earth, the level of the xe2x80x98modulationxe2x80x99 signal available at the subsurface fractures would be larger in amplitude compared to the higher frequency xe2x80x98carrierxe2x80x99 signal.
Experiments in rocks show a large nonlinear elastic wave response, far greater than that of gases, liquids, and most other solids, The large response is attributed to structural discontinuities in the rocks such as fractures (P. A. Johnson and K. R. McCall, Los Alamos National Laboratory, Los Alamos, N.Mex.). Two compressional waves, as they propagate through a fractured rock that acts as an elastically nonlinear medium, interact with each other. Due to this nonlinear interaction, the sum and difference frequencies of the two primary waves are created. These new frequencies constitute the xe2x80x98interactionxe2x80x99 wave that travels along with the primary waves. The amplitude of the summed frequencies or the xe2x80x98interactionxe2x80x99 wave is a function of the amplitudes of the two primary waves and the propagation distance through the nonlinear rock. The amplitude of the xe2x80x98interactionxe2x80x99 wave is proportional to the product of the primary wave amplitudes. Its amplitude grows with propagation distance due to nonlinearity, and decays with distance due to attenuation. Reference U.S. Pat. No. 6,175,536 (Khan), where the interaction of the two crosswell seismic signals was successfully recorded and displayed as they propagate through the nonlinear reservoir formations.
This invention uses the measurement of the summed and differenced frequencies that are created due to the interaction of the two seismic (waves) or signals as they propagate through the open fractures in the reservoir formation. One of the signals is a vibratory xe2x80x98sweepxe2x80x99 commonly used for seismic recording; the frequency is swept over the seismic band from low to high or high to low over a period of several seconds. The concept is well known in the industry and is the current art.
The second signal is a mono-frequency sinusoidal signal, which has the same time duration as the vibratory xe2x80x98sweepxe2x80x99. Both the seismic signals or waves are generated, and transmitted using standard vibratory sources from a single source array, that behave as a single surface source location The combined seismic wave is used for seismic reflection recording. It propagates through the surface formations and is transmitted and reflected at the formation boundaries that provide acoustic impedance contrasts. The reflected seismic signals are recorded using multiple detector arrays, located on the surface or in different wellbores or both. The recording procedures are known in the current art.
In this invention, the interaction of the two compressional seismic waves as they propagate through the reservoir rocks is measured to map their nonlinear characteristics that are caused due to the open fractures in the rocks. The data, which are recorded, have two different sets of information. The cross-correlation with the standard xe2x80x98sweepxe2x80x99 provides the normal data-set that is used for normal reflection processing similar to current 2-D and 3-D seismic processing; it is universally practiced and known in the art. The second set of information is extracted by generating two new xe2x80x98sweepxe2x80x99 signals. These new signals are synthetically generated by adding and differencing the mono-frequency with the xe2x80x98sweepxe2x80x99 frequencies, thus providing two xe2x80x98modified-sweepsxe2x80x99 and cross-correlating the recorded data with these xe2x80x98modified-sweeps.xe2x80x99
This new set of data, which results after cross-correlation with the two xe2x80x98modified-sweepsxe2x80x99 and contains newly generated frequencies, represents the result of interaction between the mono-frequency wave and the xe2x80x98sweepxe2x80x99 frequency wave, as they propagate through the nonlinear fractured reservoir rocks. The processing parameters for this new data-set are similar to the parameters used for the data generated after cross correlation with the primary xe2x80x98sweepxe2x80x99 signal. Conventional 2-D and 3-D seismic processing sequence can be used for both sets of data to provide the reflection seismic image of the subsurface. The integration and interpretation of the two results, one based on the primary xe2x80x98sweepxe2x80x99, and the other based on the two xe2x80x98modified-sweepsxe2x80x99, highlights and identifies the subsurface formations that are nonlinear due to open fractures. The results based on the two xe2x80x98modified-sweepsxe2x80x99 will display the reflected signals from the fractured formations at relatively higher amplitudes compared to the reflections from homogeneous and non-porous formations. The relative amplitudes of different reflections determine the relative measure of the bulk fractures in the subsurface formations.
The seismic results based on the second data-set that are produced after cross-correlation with the two xe2x80x98modified-sweepsxe2x80x99 identify and highlight the zones that have higher nonlinearity due to open fractures.
When there are open fractures in a reservoir formation, the amplitude of the reflected signal from the lower boundary of the fractured formation will be affected by the presence of fractures, their extent and their orientation. The amplitude of the data that results after the cross-correlation with the xe2x80x98modified-sweepsxe2x80x99 will vary with the source/receiver azimuth and will depend on the direction of travel of the seismic transmitted and reflected signal relative to the fractures location and orientation. Relatively larger amplitudes will signify that the seismic signal and the xe2x80x98carrierxe2x80x99 and xe2x80x98modulationxe2x80x99 waves are arriving at the fractures at a wide angle, and the direction of arrival is at right angles to the fractures.
3-D seismic data can be easily sorted according to the source/receiver azimuthal angles and offset distances; this is done routinely as a part of data processing, and is well known in the industry. The sorted data provides 360 degrees azimuth control and offset distance distribution that can vary from near zero distance to several thousand feet. This wide range of offset distances and azimuthal angles can be used for fracture detection, using the cross-correlated results of the xe2x80x98modified-sweeps.xe2x80x99
3-D seismic data volume that results after cross-correlation with the xe2x80x98modified-sweepxe2x80x99 can be sorted according to the source/receiver azimuthal angles and the 2-D seismic data (slices) that result are processed for 2-D reflection imaging. The quality and the amplitude of the reflected signals, which correspond to the fractured formation, are analyzed on all the 2-D slices. The 2-D reflected image of the fractured formation that shows the highest coherency and amplitude indicates that the particular azimuthal angle is perpendicular to the fracture orientation and the lowest amplitude represents the azimuthal angle parallel to the fracture orientation. The common mid-point of the highest amplitude reflected signal will identify the location of the fractures.